by adaptation to the peculiar conditions of embryonic life (within the fœtal membranes). In view of these facts, we may now give the following more precise expression to our chief law of biogeny: The evolution of the fœtus (or _ontogenesis_) is a condensed and abbreviated recapitulation of the evolution of the stem (or _ phylogenesis_); and this recapitulation is the more complete in proportion as the original development (or _palingenesis_) is preserved by a constant heredity; on the other hand, it becomes less complete in proportion as a varying adaptation to new conditions increases the disturbing factors in the development (or cenogenesis). [3] All these, and the following structures, will be fully described in later chapters.—Translator. The cenogenetic alterations or distortions of the original palingenetic course of development take the form, as a rule, of a gradual displacement of the phenomena, which is slowly effected by adaptation to the changed conditions of embryonic existence during the course of thousands of years. This displacement may take place as regards either the position or the time of a phenomenon. The great importance and strict regularity of the time-variations in embryology have been carefully studied recently by Ernest Mehnert, in his _Biomechanik_ (Jena, 1898). He contends that our biogenetic law has not been impaired by the attacks of its opponents, and goes on to say: “Scarcely any piece of knowledge has contributed so much to the advance of embryology as this; its formulation is one of the most signal services to general biology. It was not until this law passed into the flesh and blood of investigators, and they had accustomed themselves to see a reminiscence of ancestral history in embryonic structures, that we witnessed the great progress which embryological research has made in the last two decades.” The best proof of the correctness of this opinion is that now the most fruitful work is done in all branches of embryology with the aid of this biogenetic law, and that it enables students to attain every year thousands of brilliant results that they would never have reached without it.